1. Field of the Invention
The present invention relates to a method of forming bumps and a device for metal plating.
2. Description of the Related Art
As a method of connecting semiconductor chips with a substrate, there are known a wire bonding method, a TAB method and a flip-chip connecting method.
Since the flip-chip connecting method satisfies a demand for an increased number of connecting pin points and a decreased the signal propagation delay time, it has gradually come into wide use. Especially, because the flip-connecting method, in which solder bumps are used, is capable of forming a large number of connections all at once, it has come into very wide use. Examples of bump forming methods for connecting the flip-chips are: the electrical plating method, the vapor deposition method, and the stud bump forming method conducted by wire bonding.
It is advantageous to use the electrical plating method because it is simple and the cost is low.
Conventionally, when bumps are formed by the electrical plating method, the following method is adopted.
First, a metallic layer made of an under-barrier-metal such as Ti/Cu, Cr/Cu and Cr/Ni is formed all over a wafer, on which wiring of a circuit used for a large number of chips has already been provided, by means of sputtering or non-electrolytic plating.
A liquid photo-resist is coated on the metallic layer several times, so that a resist layer, the thickness of which is approximately 50 .mu.m, is formed. When this resist layer is processed by means of photolithography, fine holes are formed on the resist layer, so that a portion of the metallic layer on which bumps are formed can be exposed.
Then, solder bumps are formed on the metallic layer by the electrical plating method.
Next, the resist layer is removed and the under-barrier metal, except where the bumps are formed, is removed by means of etching.
FIG. 11 is an arrangement view showing an electrical plating device commonly used for the above electrical plating method.
Reference numeral 10 is an outer tank, and reference numeral 12 is a cup facing upward. Reference numeral 14 is an anode plate horizontally arranged in a lower portion of the cup 12. Reference numeral 16 is a plating solution jet pipe which penetrates the outer tank 10 and the anode plate 14, and an end of the plating solution jet pipe is open to the lower portion of the cup 12. Reference numeral 18 is a holding jig capable of holding a wafer 20 while it is electrically connected with the wafer 20. As can be seen in the drawing, the holding jig 18 is arranged at an open portion of the cup 12 while the surface of the wafer 20 to be plated is set downward. This holding jig 18 is also used as a cathode.
As shown in the drawing, the plating solution is fed from the jet pipe 16 into the cup 12 and further jets out toward the surface of the wafer 20 to be plated. When both electrodes are energized with electric current, bumps can be formed as described before.
The plating solution overflows from a gap formed between the holding jig 18 and the brim of the cup 12 and drops into the outer tank 10. After that, the plating solution returns to the tank via the discharge pipe 22.
FIG. 12 is a view showing a profile of the bump 24 formed by the above electrical plating device.
In this connection, when the bumps are formed by the above conventional electrical plating method, the following problems may be encountered.
In the above electrical plating device, plating is conducted as follows. An 8 inch diameter wafer, having a large number of fine holes, for example, about 400,000 fine holes, is attached to the electrical plating device while the wafer is set downward, and the plating solution is jetted onto the wafer from a lower portion. Due to the above arrangement, air tends to remain in the fine holes. As a result, there are holes in which plating is not conducted at all or plating is conducted insufficiently. Therefore, the product yield is deteriorated. Especially, in the case of a wafer, it is sized so that it can be formed into small narrow chips. Accordingly, the product yield of semiconductor chips is further deteriorated.
One reason for the occurrence of variation in the formation of a plating layer is a difference in the flow rate of the plating solution between the center and the periphery. Since the brim of the cup 12 is open to the outer tank 10, the flow rate of the plating solution on the peripheral part, which overflows into the outer tank 12 of low resistance, is higher than the flow rate of the plating solution on the central part. Due to the foregoing, the thickness of the plating layer on the central part tends to be larger than that on the peripheral part.
Conventionally, it is impossible to form a plating layer of large thickness because the holes are fine in the case of a wafer. In order to solve the above conventional problem, a resist layer, the thickness of which is approximately 50 .mu.m, is formed, and the solder bump 24 is formed into a mushroom shape as shown in FIG. 12 so that a lack of height and quantity can be made up.
However, when the solder bump 24 is formed into a mushroom shape, the diameter of the solder bump 24 is increased. In accordance with an increase in the diameter, it becomes difficult to form a pattern densely, which is opposite to the demand for an increased number of pins. Further, an upper portion of the mushroom-shaped bump tends to collapse. Therefore, it is difficult for the mushroom-shaped bumps to be subjected to the KGD (Known Good Die) electrical continuity test in which the bumps are pressed against the inspection wiring so as to check the electrical continuity.
Since the diameter of the solder bump is changed in the process of electric plating, that is, the diameter of the solder bump is gradually increased in the process of electric plating, a change is caused in the density of electric current. Therefore, in the case of solder plating, there is a possibility that the composition of solder is changed. For this reason, it is necessary to adjust the density of electric current to be constant, which is troublesome, and further it becomes necessary to provide an expensive device to adjust the density of electric current.